Abstract:
A simple model for the bottom boundary layer on the continental shelf
is presented. The governing equations are developed for a stratified,
turbulent Ekman layer in a combined wave and current flow over a moveable
sediment bed. An eddy diffusivity closure scheme that includes the
effect of suspended sediment, temperature, and salinity induced
stratification on the vertical turbulent diffusion of mass and momentum
couples the resulting unsteady conservation equations for fluid momentum,
fluid mass, and suspended sediment mass. The wave velocity, current
velocity, and suspended sediment concentration profiles predicted by the
simultaneous solution of the conservation equations require the physical
bottom roughness and a sediment reference concentrati on to be specified
as boundary conditions. The physical bottom roughness associated with
biologically generated bedforms, wave generated ripples, and near bed
sediment transport are calculated as functions of the flow and sediment
conditions. Using expressions for the height of sediment transporting
layer and the sediment velocity, an expression for the sediment reference
concentration is developed by matching laboratory measurements of
sediment transport rates in oscillatory flow. The model predicts that
the bottom flow field is highly dependent on (1) the nonlinear wave and
current interaction, which increases the boundary shear stress and
enhances vertical turbulent diffusion, (2) the effect of the boundary
shear stress on a moveable sediment bed, which determines the physical
bottom roughness and the amount of sediment in suspension, and (3) the
effect of stable stratification, which inhibits vertical turbulent
transport and couples the flow to the suspended sediment and fluid
density profiles. The validity of the theoretical approach is supported
by model predictions that are in excellent agreement with high quality
data collected during two continental shelf bottom boundary layer
experiments for a wide range of flow and bottom conditions.
